1. Field of the Invention
The invention described herein relates to injection or transfer molding method and device in which raw material of rubber, rubber like substances or synthetic resins is injected into a mold to obtain hardened or cured products by vulcanization or intermolecular cross linkage reaction.
2. Description of the Prior Art
This invention aims at providing a method and a device to shorten the period required for vulcanization or hardening. The following description will be limited to rubber, although this invention is applicable to other materials such as rubber like substances and synthetic resins.
The ultimate object of the injection molding of rubber is considered to be, "to vulcanize the raw material as soon as it is injected into the mold cavity." Methods employed hitherto, however, are far from the above objective, although much effort has been devoted toward achieving this object. For example, conventional vulcanization takes at least 2 minutes after raw material is injected into the mold cavity. Usually a 4 to 10 minutes period is required for vulcanization in the mold cavity at about 150.degree. C. a conventional molding temperature.
Before coming into the details of the prior art, thermal characteristics of rubber will be described below. After adding necessary elements such as vulcanizing agent and after mastication, the vulcanization process of raw rubber depends on the heat history in which temperature and time are parameters. Rate of vulcanization is quite sensitive to its temperature. For example, temperature increase of 10.degree. C. decreases the vulcanization period to about a half.
From this character of raw rubber, it can be said theoretically that vulcanization finishes within 15 seconds when the raw material temperature in the mold cavity is, for example, 180.degree. C. Usually, raw rubber is treated in a low temperature for example between 75.degree. C. and 120.degree. C. before injection in order to avoid scorching. Once rubber is scorched, it loses its flowability irreversibly. Therefore, in the conventional method, temperature of the raw rubber is kept low enough before injection and is heated to a vulcanization temperature after injected into the mold cavity. That is, in the conventional method, both heat-up time and vulcanization time are required in the mold cavity. This is one of the reasons why the vulcanization period can not be shortened in the conventional method.
There have been some efforts to raise the temperature of the raw rubber just before injected into the mold cavity in order to save the heat-up time in the mold cavity. However, the heating method in the conventional injection or transfer molding is adding heat to the raw material through structures during the process before injection, such as in the process of compressing, heading, heat softening, transfer, etc. That is, the heaters are located outside of thick structures or flow paths of heat medium are located in the thick structures.
As heat is transfered through thick structures in the conventional method, temperature response of raw material to the heater is very slow. This made it very difficult to raise the temperature of raw material sharply just before it is injected into the mold.
In the injection molding process, a predetermined mass of raw material is transfered intermittently. Heat capacity of the structures and mass of raw material which is heated at a time are large in the conventional method. This makes the time necessary to achieve the thermal equilibrium between the raw material and the heating device long. This is one of the disadvantages of the conventional method. That is, degree of vulcanization of material which is heated at an early stage of injection is different from that of material heated at a later stage as they have different heat history. This creates nonuniformity of the products.
Rubber is rather a heat insulator than a good heat conductor. The heat conductance of rubber, for example, is 1/500 of steel, 1/5 of water and 6 times that of air. The heat capacity of rubber is more than 4 times that of steel, although it depends on the compounding ratio of the rubber. From this character of the rubber, another disadvantage of the conventional method arises. That is, raw material is heated from surroundings after injected into the mold in the conventional method and this requires the time for heat to be transferred to the inside of the products. And the difference in heat history between surface and core makes the product nonuniform. Nonuniformity of vulcanization is a serious problem especially in cases where the product is thick or has uneven thicknesses.